Because h2o when in liquid form tends to flow to the oceans, which makes their levels rise. If it doesn't make it to the ocean, it becomes vapor and then falls as ran, and since 2/3 of the world is ocean, it eventually becomes part of the ocean. Which tends to make the sea levels rise. The sea levels were hundreds of feet lower during the last ice age, and rose quite a bit after it ended, deluging human villages and settlements all over the globe. Warmists believe that global warming, and melting of ice that is currently on top of land masses, will cause the seas to rise even higher than they are at the moment, flooding New York and other cities along the coast. More likely than not, we will get another ice age, as one is overdue. Periods of warming between ice ages are the exception, rather than the norm, over the last few million years.
Comprende?
To address your point about arctic sea ice, if it melts, that would not change sea levels a great deal. But I think most of it melts every year any way. The warmists worry about ice on the land. You make a valid point about sea ice, but I don’t know how much sea ice enters into the equation.